Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light. Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes and, in some cases, significantly higher efficiency for converting electric energy to light.
The conversion efficiency of individual LEDs is an important factor in addressing the cost of high power LED light sources. The conversion efficiency of an LED is defined to be the electrical power dissipated per unit of light that is emitted by the LED. Electrical power that is not converted to light in the LED is converted to heat that raises the temperature of the LED. The light conversion efficiency of an LED decreases with increasing current through the LED.
LEDs are typically powered from a DC power source or a modulated square wave source so that a constant current flows through the LED while the LED is “on”. The current is set to provide high efficiency. In light sources with variable intensity, the intensity of the light is controlled by changing the duty factor of the modulated square wave so that the current flowing through the LED is at a value consistent with providing the desired efficiency.
Conventional lighting systems typically must be powered from an AC power source. Hence, an LED-based light source typically includes an AC-DC power converter. The cost of the power converter represents a significant fraction of the cost of a typical LED light source. In addition, the power losses in the power converter reduce the overall efficiency of the light source.
To avoid these costs, LED light sources that operate directly from an AC power source without the power first being converted to DC have been proposed. Such light sources typically include two strings of LEDs. The LEDs are connected in series in each string. One string is powered on when the AC waveform is in the positive half of the sine wave, and the other is powered when the AC waveform is in the negative half of the sine wave.
This simple driving scheme suffers from low efficiency and flicker. Consider a single LED that is driven by an AC waveform. In general, the LED is characterized by a minimum voltage that must be applied to forward bias the LED so that a current will flow through the LED. During the half of the AC cycle in which the diode is forward biased, the LED will remain off until the sine wave reaches this voltage. During the portion of the sine wave in which the LED is on, the average current must be set to the optimum current from a power efficiency point of view. Hence, during a portion of the cycle, the current will be higher than the optimum power, and the efficiency of the LED will be reduced. During the portion of the sine wave in which the voltage is less than that required to turn on the LED, the LED will be dark. This gives rise to a flicker in the intensity at a frequency that is twice the frequency of the AC light source.
In a co-pending application, U.S. Ser. No. 12/504,994, filed on Jul. 17, 2009, an improved AC LED light source is described in which each LED in a series string is connected in parallel with a switch that shorts that LED when the AC voltage across the string is insufficient to drive all of the LEDs in the string. In this manner, the LEDs that remain are driven with a current more nearly equal to the optimum current, and hence, the efficiency losses described above are reduced. While this arrangement improves the overall conversion efficiency, the resultant light source still suffers from flicker. In addition, the average number of LEDs that are powered over the AC voltage cycle is low, and hence, the number of LEDs needed to provide a predetermined light output is increased relative to DC driven LED light sources.